Laser detection and ranging (ladar) may be used in a military context for elastic backscatter light detection and ranging (lidar) systems. The acronym LADAR is usually associated with the detection of hard targets, while the acronym LIDAR is usually associated with the detection of aerosol targets. However, there has been no real standard on their use and both acronyms may be used interchangeably to describe the same laser ranging system. A ladar system is similar to a radar system with the exception that a much shorter wavelength of the electromagnetic spectrum is used, typically in the ultraviolet, visible, or near infrared spectrums. With a radar system, it is possible to image a feature or object of about the same size of the wavelength or larger. Due to diffraction limits, the laser radiation is easier to collimate than microwave radiation given realistic aperture constraints. This gives a compact ladar the ability to image a target with at a high spatial resolution.
In order for a ladar system target to reflect a transmitted electromagnetic wave, an object needs to produce a dielectric discontinuity from its surroundings. At radar frequencies, a metallic object produces a dielectric discontinuity and a significant specular reflection. However, non-metallic objects, such as rain and rocks produce weaker reflections, and some materials may produce no detectable reflection at all, meaning some objects or features are effectively invisible at radar frequencies. In a military context, metallic objects may be disguised by use of a non-metallic covering material.
Lasers provide one solution to this problem regarding non-metallic detection. The beam densities and coherency of lasers are excellent. Moreover, the wavelengths are much smaller than can be achieved with radio systems, and range from about 10 μm to around 250 nm. At such wavelengths, the waves are reflected very well from small objects such as molecules and atoms. This type of reflection is called diffuse “backscattering.” Both diffuse and specular reflection may be used for different ladar applications.
The Army Research Laboratory has demonstrated the capability of its previously patented chirped amplitude modulation (AM) ladar technique to produce high-resolution, range-resolved intensity imagery (3D+intensity) of targets in heavy clutter, under dense foliage canopy, and obscured by camouflage nets. This technique is covered in U.S. Pat. No. 5,877,851 for “Scannerless Ladar Architecture Employing Focal Plane Detector Arrays and FM-CW Ranging Theory” and in U.S. Pat. No. 5,608,514 for “High-Range Resolution Ladar,” which are hereby incorporated by reference. These previously patented techniques use a modulated continuous wave (CW) laser as an illumination source and a photon detection receiver. For some applications, a high-peak power illumination source is required to satisfy the design requirements, which is not easily achieved with a CW source. For those applications, it is advantageous to use a short-pulse laser as an illumination source due to their high-peak powers and commercial availability. However, since the duration of the laser pulse is extremely short, it is not feasible to chirp modulate the laser intensity as is required with the current ladar architecture Therefore, a modification to the existing ranging technique is required.